1. Field of the Invention
The present invention pertains to electron exposure equipment useful for exposing, treating and processing coatings and other materials. More specifically, the invention relates to a cold cathode gas discharge electron source having a broad uniform emitting area.
2. Description of the Related Art
In the development of many processes for treating materials, especially in producing semiconductor devices, there is a need for a broad electron beam exposure area. In this regard, a need exists in the art for a large-area electron beam source which is controllable, uniform, insensitive to poor vacuum and long lived. Such electron beams are used for curing interlayer dielectrics for microelectronic devices, photoresist exposure, altering solubility characteristics of thin film layers, and the like. These processes require an electron beam source which can be controlled in voltage and current, and can withstand the outgassing of the material being irradiated. In the past, electron beam sources used to treat large volumes of materials have typically been isolated by means of a vacuum window to protect the electron emitting cathode from outgases from the material being treated. Due to the difficulty of making large-area, electron permeable vacuum windows, these systems have utilized a small scanning beam which is then raster scanned to uniformly expose larger areas. To achieve higher throughput processing, it is advantageous to use a large-area or flood beam electron source, to expose the whole substrate simultaneously, rather than raster scanning.
Large-area thermionic cathodes are known, however these require a good vacuum environment to be long lived, and also generate an excess of heat, which can adversely affect the material being processed. For processes in which the substrate is temperature sensitive, a cold cathode is desirable. Cold cathode electron sources are also known. An electron beam system utilizing a large-area gas discharge source is described in U.S. Pat. No. 4,496,449. U.S. Pat. No. 4,119,688 describes a pulsed glow discharge system, with which it is difficult to attain uniform exposure and precise dose control. Another large-area cathode, based on photoemission, is disclosed in U.S. Pat. No. 4,554,458, However, photocathodes are easily poisoned and require an ultraclean high vacuum environment.
It will be appreciated from the foregoing that there is a significant need for an improved electron source that will overcome many of the disadvantages of the aforementioned earlier electron beam sources. In particular, what is needed is a broad area, uniform, cold, electron beam source with continuously variable voltage, capable of operation in a soft vacuum. Ideally, the electron source should also have a continuously variable accelerating voltage that can be utilized to rapidly process large-area substrates in semiconductor device fabrication for high resolution shadow mask lithography, photoresist curing or hardening, blanket exposure of resists for aiding liftoff processes, enhancing resist contrast or controlling resist pattern dimensions or pattern edge profiles and to provide a highly uniform e-beam flux to provide consistent results to the treated film.
U.S. Pat. No. 5,003,178, which is incorporated herein by reference, describes a large area uniform electron source which provides a fairly uniform electron beam over large areas. It has been found in the application of this apparatus and the further development of integrated circuit processes, that a higher degree of uniformity is required to meet the needs of next generation microelectronic devices. In the generation of an electron beam as described in U.S. Pat. No. 5,003,178, there a fall-off in beam intensity from the center to the edge of the source. This fall-off, which is on the order of 25-30%, is caused by a lower density of ions on the periphery of the beam. This results in a non-uniformity in the exposure of a target by the beam. In the device of U.S. Pat. No. 5,003,178, there is an anode between the cathode and the target substrate which separates the cathode and the accelerating field from the target. The anode has a uniform density of holes in this anode grid. The present invention discloses methods for correcting beam intensity fall-off from the center to the edge. As a result a uniformity of better than xc2x12.5% over the full emitting area of the source can be attained.
The invention provides an electron emission apparatus, comprising:
(a) a vacuum chamber;
(b) a large surface area cathode in the vacuum chamber;
(c) means for applying a negative voltage to the cathode and causing the cathode to issue electrons toward a target in the vacuum chamber;
(d) an anode spaced apart from the cathode and positioned between the cathode and the target; said anode comprising an electrically conductive grid having an array of spaced apart apertures therethrough extending from a center of the grid to an edge of the grid; said apertures having progressively increasing area from the center of the grid to the edge of the grid; and
(e) means for applying a voltage to the anode which is positive relative to the voltage applied to the cathode.
The invention also provides an electron emission apparatus, comprising:
(a) a vacuum chamber;
(b) a large surface area cathode in the vacuum chamber;
(c) means for applying a negative voltage to the cathode and causing the cathode to issue electrons toward a target in the vacuum chamber;
(d) an anode spaced apart from the cathode; and positioned between the cathode and the target; said anode comprising an electrically conductive grid having a progressively decreasing thickness from a center of the grid to an edge of the grid; and having an array of spaced apart apertures therethrough extending from the center of the grid to the edge of the grid; and
(e) means for applying a voltage to the anode which is positive relative to the voltage applied to the cathode.
The invention further provides an electron emission apparatus, comprising:
(a) a vacuum chamber;
(b) a large surface area cathode in the vacuum chamber;
(c) means for applying a negative voltage to the cathode in an amount sufficient to drive electrons toward a target in the vacuum chamber;
(d) an anode spaced apart from the cathode and positioned between the cathode and the target; said anode comprising an electrically conductive grid having an array of spaced apart apertures therethrough extending from a center of the grid to an edge of the grid; said apertures having progressively increasing area from the center of the grid to the edge of the grid; and
(e) means for applying a voltage to the anode which is positive relative to the voltage applied to the cathode; and
(f) means for generating electrons between the cathode and the anode.
The invention still further provides an electron emission apparatus, comprising:
(a) a vacuum chamber;
(b) a large surface area cathode in the vacuum chamber;
(c) means for applying a negative voltage to the cathode in an amount sufficient to drive electrons toward a target in the vacuum chamber;
(d) an anode spaced apart from the cathode; and positioned between the cathode and the target; said anode comprising an electrically conductive grid having a progressively decreasing thickness from a center of the grid to an edge of the grid; and having an array of spaced apart apertures therethrough extending from the center of the grid to the edge of the grid; and
(e) means for applying a voltage to the anode which is positive relative to the voltage applied to the cathode; and
(f) means for generating electrons between the cathode and the anode.
The invention further provides a charged particle emission apparatus, comprising:
(a) a vacuum chamber;
(b) a first electrode capable of directing charged particles in the vacuum chamber;
(c) means for applying a voltage to the cathode and causing the first electrode to direct charged particles toward a target in the vacuum chamber;
(d) a second electrode spaced apart from the first electrode and positioned between the first electrode and the target; said second electrode comprising an electrically conductive grid having an array of spaced apart apertures therethrough extending from a center of the grid to an edge of the grid; said apertures having progressively increasing areas from the center of the grid to the edge of the grid; and
(e) means for applying a voltage to the second electrode which attracts the charged particles directed by the first electrode.
The invention still further provides a charged particle emission apparatus, comprising:
(a) a vacuum chamber;
(b) a first electrode capable of directing charged particles in the vacuum chamber;
(c) means for applying a voltage to the cathode and causing the first electrode to direct charged particles toward a target in the vacuum chamber;
(d) a second electrode spaced apart from the first electrode and positioned between the first electrode and the target; said second electrode comprising an electrically conductive grid having a progressively decreasing thickness from a center of the grid to an edge of the grid; and having an array of spaced apart apertures therethrough extending from the center of the grid to the edge of the grid; and
(e) means for applying a voltage to the second electrode which attracts the charged particles directed by the first electrode.
The invention still further provides a method for producing a highly uniform electron beam emission, comprising:
(a) applying a negative voltage to a large-area cathode positioned in a vacuum chamber, to direct electrons in the vacuum chamber toward a target in the vacuum chamber;
(b) applying a voltage to an anode spaced apart from the cathode and positioned between the cathode and the target, which voltage is positive relative to the voltage applied to the cathode; said anode comprising an electrically conductive grid having an array of spaced apart apertures therethrough extending from a center of the grid to an edge of the grid; said anode having one or both of (i) and (ii):
(i) apertures having progressively increasing area from the center of the grid to the edge of the grid;
(ii) a progressively decreasing thickness from a center of the grid to an edge of the grid.
The invention still further provides a method for producing a highly uniform charged particle emission comprising:
(a) applying a voltage to a large-area cathode positioned in a vacuum chamber and causing the first electrode to direct charged particles toward a target in the vacuum chamber;
(b) applying a voltage to a second electrode spaced apart from the first electrode and positioned between the first electrode and the target; which voltage attracts the charged particles directed by the first electrode; said second electrode comprising an electrically conductive grid having an array of spaced apart apertures therethrough extending from a center of the grid to an edge of the grid; said second electrode having one or both of (i) and (ii):
(i) apertures having progressively increasing area from the center of the grid to the edge of the grid;
(ii) a progressively decreasing thickness from a center of the grid to an edge of the grid.